The present invention relates to several methods for efficient production of oxygen by cryogenic air separation. In particular, the present invention relates to cryogenic air separation processes where it is attractive to produce at least a portion of the total oxygen with purity less than 99.5% and, preferably, less than 97%.
There are numerous U.S. patents that teach the efficient production of oxygen with purity less than 99.5%. Two examples are U.S. Pat. Nos. 4,704,148 and 4,936,099.
U.S. Pat. No. 2,753,698 discloses a method for the fractionation of air in which the total air to be separated is prefractionated in the high pressure column of a double rectifier to produce a crude (impure) liquid oxygen (crude LOX) bottoms and a gaseous nitrogen overhead. The so produced crude LOX is expanded to a medium pressure and is completely vaporized by heat exchange with condensing nitrogen. The vaporized crude oxygen is then slightly warmed, expanded against a load of power production and scrubbed in the low pressure column of the double rectifier by the nitrogen condensed within the high pressure column and entered on top of the low pressure column. The bottom of the low pressure column is reboiled with the nitrogen from the high pressure column. This method of providing refrigeration will henceforth be referred to as CGOX expansion. In this patent, no other source of refrigeration is used. Thus, the conventional method of air expansion to the low pressure column is replaced by the proposed CGOX expansion. As a matter of fact, it is cited in this patent that the improvement results because additional air is fed to the high pressure column (as no gaseous air is expanded to the low pressure column) and this results in additional nitrogen reflux being produced from the top of the high pressure column. It is stated that the amount of additional nitrogen reflux is equal to the additional amount of nitrogen in the air that is fed to the high pressure column. An improvement in the efficiency of scrubbing with liquid nitrogen in the upper part of the low pressure column is claimed to overcome the deficiency of boil-up in the lower part of the low pressure column.
U.S. Pat. No. 4,410,343 discloses a process for the production of low purity oxygen which employs a low pressure and a medium pressure column, wherein the bottoms of the low pressure column are reboiled against condensing air and the resultant air is fed into both the medium pressure and low pressure columns.
U.S. Pat. No. 4,704,148 discloses a process utilizing high and low pressure distillation columns for the separation of air to produce low purity oxygen and a waste nitrogen stream. Feed air from the cold end of the main heat exchangers is used to reboil the low pressure distillation column and to vaporize the low purity oxygen product. The heat duty for the column reboil and oxygen product vaporization is supplied by condensing air fractions. In this patent, the air feed is split into three substreams. One of the substreams is totally condensed and used to provide reflux to both the low pressure and high pressure distillation columns. A second substream is partially condensed with the vapor portion of the partially condensed substream being fed to the bottom of the high pressure distillation column and the liquid portion providing reflux to the low pressure distillation column. The third substream is expanded to recover refrigeration and then introduced into the low pressure distillation column as column feed. Additionally, the high pressure column condenser is used as an intermediate reboiler in the low pressure column.
In international patent application #PCT/US87/01665 (U.S. Pat. No. 4,796,431), Erickson teaches a method of withdrawing a nitrogen stream from the high pressure column, partially expanding this nitrogen to an intermediate pressure and then condensing it by heat exchange against either crude LOX from the bottom of the high pressure column or a liquid from an intermediate height of the low pressure column. This method of refrigeration will now be referred to as nitrogen expansion followed by condensation (NEC). Generally, NEC provides the total refrigeration need of the cold box. Erickson teaches that only in those applications where NEC alone is unable to provide the refrigeration need, that supplemental refrigeration is provided through the expansion of some feed air. However, use of this supplemental refrigeration to reduce energy consumption is not taught. This supplemental refrigeration is taught in the context of a flowsheet where other modifications to the flowsheets were done to reduce the supply air pressure. This reduced the pressure of the nitrogen to the expander and therefore the amount of refrigeration available from NEC. In this patent, Erickson also teaches the use of two NEC. The nitrogen from the high pressure column is split into two streams, and each stream is partially expanded to different pressures and condensed against different liquids. For example, one expanded nitrogen stream is condensed against crude LOX and the other is condensed against an intermediate height liquid from the low pressure column. Erickson claims that the use of a second NEC increases the refrigeration output that can be used to power a cold compressor so as to further increase oxygen delivery pressure.
In U.S. Pat. No. 4,936,099, Woodward et al use CGOX expansion in conjunction with the production of low purity oxygen. In this case, gaseous oxygen product is produced by vaporizing liquid oxygen from the bottom of the low pressure column by heat exchange against a portion of the feed air.
In some air separation plants, excess refrigeration is naturally available. This is generally for either of two reasons: an operating equipment constraint leads to excess flow through the expander, and recovery of the product from the distillation system is low and it produces excess waste at an elevated pressure which is then expanded. In such cases, some patents have suggested to use excess refrigeration for compressing a suitable process stream at cryogenic temperatures. This method of compression at cryogenic temperatures will hereinafter be referred to as cold compression.
An example of the creation of excess refrigeration due to the first reason and then use of cold compression can be found in U.S. Pat. No. 4,072,023. In this patent, reversing heat exchangers are used to remove water and carbon dioxide from the feed air. A successful operation of such a reversing heat exchanger requires that a balance stream be used. The balance stream is generally drawn from the distillation column system, then partially warmed in the cold part of the main heat exchanger in indirect heat exchange with the incoming feed air, and then expanded in an expander to provide the needed refrigeration. Unfortunately, the flow rate of this balance stream cannot be reduced below a certain fraction of the feed air flow rate. For large size plants where the refrigeration demand per unit of product flow is not that large, the constraint of having a balance stream flow above a certain fraction of the feed air flow produces excess refrigeration. In this patent, a predominantly nitrogen containing or a predominantly oxygen containing cold stream from a double column process is expanded in an expander. Some of the work energy from this expander is used to compress a process stream which is at a temperature between that of the double distillation column and the cold end of the main heat exchanger. This patent teaches this cold compression scheme in context with a conventional double column process where the top of a high pressure column is thermally linked with the bottom of the low pressure column.
Examples of the creation of excess refrigeration due to the second reason and then use of cold compression can be found in U.S. Pat. Nos. 4,966,002 and 5,385,024. In both of these patents, air is fed near the bottom of a single distillation column to produce high pressure nitrogen. Since a single distillation column with no reboiler at the bottom is used, the recovery of nitrogen is low. This produces a large quantity of oxygen-enriched waste stream at an elevated pressure. A portion of this oxygen-enriched waste stream is partially warmed and expanded to provide the needed refrigeration, and the excess refrigeration is used to cold compress another portion of this waste stream. The cold compressed waste stream is recycled to the distillation column.
In U.S. Pat. No. 5,475,980, cold compression is used to improve the efficiency of cooling in the heat exchanger vaporizing pumped liquid oxygen at a pressure greater than about 15 bar. For this purpose, an auxiliary stream at an intermediate temperature is taken out from an intermediate location of the heat exchanger. This auxiliary stream is then cold compressed and reintroduced in the heat exchanger and further cooled. At least a portion of the further cooled stream is then expanded in an expander. When the pressure of the auxiliary stream to be cold compressed is much higher than the high pressure column pressure, then only a portion of it is expanded to the high pressure column after cold compression and partial cooling. In this case, extra energy is provided at the warm end of the plant to meet the refrigeration and cold compression requirement. However, when the auxiliary stream is withdrawn from the high pressure column, then all of it is expanded after cold compression and cooling. This ensures that most of the energy needed for cold compression is recovered from the expander and used for cold compression. As a result, the need for extra vapor flow through the expander to create work energy is minimal and it does not require excess refrigeration as in the earlier cited U.S. Pat. Nos. 4,072,023; 4,966,002 and 5,385,024.
In DE-28 54 508, a portion of the air feed at the high pressure column pressure is further compressed at the warm level by using work energy from the expander providing refrigeration to the cold box. This further compressed air stream and is then partially cooled and expanded in the same expander that drives the compressor. In this scheme, the fraction of the feed air stream which is further compressed and then expanded for refrigeration is the same. As a result, for a given fraction of the feed air, more refrigeration is produced in the cold box. The patent teaches two methods to exploit this excess refrigeration: (a) to produce more liquid products from the cold box; (b) to reduce flow through the compressor and the expander and thereby increase flow to the high pressure column. It is claimed that an increased flow to the high pressure column would result in a greater product yield from the cold box.